Minerals and mines

Lead ore and mines

Mineralisation

The lead, zinc and copper veins on Mendip were deposited by hot mineralising
fluids (typically between 50 and 150° C) rising up from depth
and depositing various minerals as they cooled. The source of the
fluids were the deep sedimentary basins either side of the Mendips.
As the Triassic and Jurassic sediments in these basins were buried,
compacted and heated over time, some of the water in the rock was
forced out, along with any dissolved metals and migrated into the
neighbouring Carboniferous Limestone. Here the change in chemistry,
temperature and pressure led to the deposition of various minerals
including lead, zinc, and locally copper.

Types of lead ore

Lead ore is most commonly found as lead sulphide (PbS), galena, a
heavy, shiny grey metallic ore with a conspicuous cubic cleavage,
but locally pyromorphite, lead chlorophosphate (Pb5(PO4)3Cl),
was worked on Green Hill, near Charterhouse and on Blagdon Hill.
Around Charterhouse, the lead ore contained small amounts of silver.

The lead ore occurs in two types of deposit: as primary lead ore
in thin veins known as rakes, or a secondary deposit formed by weathering
of the primary lead veins.

Primary lead veins

The primary lead ore was found in thin veins cutting through the
rock. In these veins, the galena occurs as either thin layers encrusting
on the walls of the vein, or as thin bands, pockets or crystals within
the vein. The veins were always associated with other waste minerals
known as ‘gangue’, usually calcite (CaCO3)
or barytes (BaSO4). Many of these veins were very thin,
sometimes only a few centimetres wide, and often pinched and swelled
along their length, sometimes forming complex anastomosing networks
with other veins.

Secondary deposits

In many places, the lead ore occurred as large rounded ‘stones’ of
galena in a soft mixture of sand and clay infilling a fissure. These
deposits are secondary residual orebodies derived from now eroded
primary lead veins. Being insoluble, the lead ore remained in situ
as the surrounding limestone and vein calcite around them wasted
away. Over time, the fragments of lead ore accumulated at ground
level, keeping pace with the land surface as it was gradually lowered
by erosion.

These secondary mineral deposits are often associated with ‘Neptunian
dykes’, fissures in the Carboniferous Limestone infilled with
younger Triassic or Jurassic sediment. Many of the spoil tips at
Charterhouse and elsewhere have Triassic or Lower Jurassic material
in them.

The early miners at Charterhouse and elsewhere probably found a rich
bonanza of secondary lead ore deposits at or close to the surface,
which pinched out at depth to thin primary veins. When the Cornish
miners attempted to deepen the mines in the nineteenth century, the
rich near-surface secondary deposits had been worked out and all
that was left were the thin uneconomic primary veins.

Orefields

Galena was principally mined around Charterhouse, Smitham Hill, Yoxter,
Chewton Warren and Green Ore, but smaller lead veins were also worked
on Sandford Hill, near Tynings Farm, Burrington Ham and north of
Pen Hill. Little ore was found on East Mendip.